Scientists create first free-standing 3D cloak

Researchers in the United States have, for the first time,
cloaked a 3D object standing in free space, bringing the much-talked-about
invisibility cloak one step closer to reality.

Whilst previous studies have either been theoretical
in nature or limited to the cloaking of 2D objects, this study shows how
ordinary objects can be cloaked in their natural environment in all directions
and from all of an observer's positions.

Published in the New
Journal of Physics, the researchers used a method known as "plasmonic cloaking" to hide an 18-cm cylindrical tube from microwaves.

Some of the most recent breakthroughs in the field of
invisibility cloaking have focussed on using transformation-based metamaterials;
however, this new approach uses a different type of artificial material—plasmonic
metamaterials.

When light strikes an object, it rebounds off its
surface towards another direction, just like throwing a tennis ball against a
wall. The reason we see objects is because light rays bounce off materials
towards our eyes and our eyes are able to process the information.

Due to their unique properties, plasmonic
metamaterials have the opposite scattering effect to everyday materials.

"When the scattered fields from the cloak and the
object interfere, they cancel each other out and the overall effect is
transparency and invisibility at all angles of observation.

"One of the advantages of the plasmonic cloaking
technique is its robustness and moderately broad bandwidth of operation,
superior to conventional cloaks based on transformation metamaterials. This
made our experiment more robust to possible imperfections, which is
particularly important when cloaking a 3D object in free-space," said study
co-author Professor Andrea Alu.

In this instance, the cylindrical tube was cloaked
with a shell of plasmonic metamaterial to make it appear invisible. The system
was tested by directing microwaves towards the cloaked cylinder and mapping the
resulting scattering both around the object and in the far-field. The cloak
showed optimal functionality when the microwaves were at a frequency of 3.1 GHz
and over a moderately broad bandwidth.

The researchers, from the University
of Texas at Austin, have shown in previous studies that
the shape of the object is irrelevant; oddly shaped and asymmetric objects can
both be cloaked using this technique.

Moving forward, one of the key challenges for the
researchers will be to demonstrate the cloaking of a 3D object using visible
light.

"In principle, this technique could be used to cloak
light; in fact, some plasmonic materials are naturally available at optical
frequencies. However, the size of the objects that can be efficiently cloaked
with this method scales with the wavelength of operation, so when applied to
optical frequencies we may be able to efficiently stop the scattering of
micrometre-sized objects.

"Still, cloaking small objects may be exciting for a
variety of applications. For instance, we are currently investigating the
application of these concepts to cloak a microscope tip at optical frequencies.
This may greatly benefit biomedical and optical near-field measurements,"
continued Alu.